Value to Management

  • Supports floodplain safety evaluation: Stream gage monitoring helps identify flood-prone areas, and detect changes in flow patterns that could be evidence of changes in land use or climate that increase flood risk.
  • Provides data for water supply management: Information from monitoring contributes to drought and water supply management and planning, forming the basis for water resource management.
  • Comprises an essential piece of water quality monitoring: Flow rates obtained from stream gages are essential to calculating overall pollutant loads, and supports development of water quality restoration plans and minimum flow standards.

The volume of water flowing through Rhode Island’s rivers and streams is always changing—by season, with weather events, and over time—reflecting changes in water use, land use, and climate. Stream flows increase after rainfall as rainwater flows over land, into storm sewers, and through soils towards water bodies. It logically follows that stream flows increase during rainy seasons, rainy years, or due to changes in the landscape that modify how much water seeps into the ground and instead flows over the surface of the land, which increases the likelihood of flooding and decreases the natural action of water cleansing during its movement through soils.

Robert Bradley of the USGS New England Water Science Center makes a streamflow measurement— important data in the stream gage network. Photo courtesy of USGS/Dennis LeBlanc.

Variations in stream flow are a critical aspect of the physical design of stream ecosystems. High flows have the power to shape the stream channel through eroding and depositing sediment, and clearing debris such as tree branches. High flows are also necessary to provide conditions that allow some fish species to travel upstream and spawn. Low flows also play an important ecosystem function. As flow decreases, the water level drops, and the area of stream bottom habitat decreases, which defines the smallest area potentially available to stream biota throughout the year. Low flows may also result in higher water temperatures, which typically have a negative impact on the native plants and animals that live there. Freshwater flows into the ocean are also very important to anadromous fish such as river herring, shad, and American eel that spend portions of their life cycles in both fresh and saltwater bodies.

Stream flow data have a wide breadth of application and are used by managers, watershed councils, and others. A few examples include: 1) flow information is critical to public safety; 2) is used by the National Weather Service to forecast floods, and 3) helps the Federal Emergency Management Agency (FEMA) identify flood prone areas. These predictions, in turn, are used to prevent people from building or developing in areas that are likely to be frequently flooded without approved adaptation: raised buildings, increase of ground level, or shoreline protection. As Rhode Island learned in 2010, floods have serious economic impacts: FEMA spent over $79 million to assist Rhode Island individuals and businesses following the historic flood of March 2010, and out-of-pocket expenses were equally impactful..

Downstream of the Pawcatuck monitoring gage. Photo courtesy of USGS and Rich Verdi

River and stream flow data are also essential to all aspects of water resource management. These data are used in drinking water supply, drought monitoring, and water pollution control management. They also support Total Maximum Daily Load (TMDL) analyses, which determine the allowable loadings of contaminants to a waterbody. Flow data are combined with measurements of the concentration of a chemical to estimate total pollutant loadings into a waterbody, and are integral to hydrodynamic and water quality modeling that is of critical use to state managers and researchers. Stream flow is also important in determining a stream’s suitability as wildlife habitat, as well as its suitability, value, and condition for recreational opportunities. If you are planning on kayaking one of Rhode Island’s scenic rivers, you need to know how high the water is running.

Impervious Surfaces

Impervious surfaces are hardened, and often reference paved areas like roofs, roads, or parking lots that water cannot pass through. Consequently, rainfall cannot seep into the ground, but instead flows over the surface until it drains to a waterbody, storm sewer, or an area where infiltration is possible, such as a field, lawn, or forest. Flow over impervious surfaces is much faster than infiltration through the ground. In densely developed areas with high percentages of impervious surfaces, streams typically have higher volumes of water after rainfall events than less developed areas with lower percentages of impervious surface. In less developed areas, some of the rainwater will flow over land surface, but a portion will always seep into the ground, moving through the soils and into the groundwater. This is a much slower process than in more developed areas where water rushes over hardened surfaces. Streams in an area with less impervious surface get a smaller pulse of high flow immediately after a rain event with a longer period of elevated flow to follow than do streams surrounded by more impervious surfaces. Sediments also serve to filter pollutants from water before it reaches the ground water, stream, or other coastal waterbody while runoff over asphalt or concrete experiences no filtration of pollutants. An increase in impervious cover can cause very high stream flows immediately after a rain event—and potentially accelerated flooding—and higher pollution levels unless effective stormwater treatment is provided.

Pervious surfaces such as grass, soils, and ‘green roofs’ allow water to infiltrate the ground, slowing and reducing runoff and recharging groundwater. Impervious surfaces such as cement, asphalt, and roofing prevent infiltration, increasing the volume and velocity of surface runoff which carries nutrients and sediments with it. Diagram courtesy of Integration and Application Network (, University of Maryland Center for Environmental Science. Sources: Dennison, W.C., J.E. Thomas, C.J. Cain, T.J.B. Carruthers, M.R. Hall, R.V. Jesian, C.E., Wasniak, and D.E. Wilson, 2009. Shifting Sands: Environment and cultural change in Maryland’s Coastal Bays. IAN Press, University of Maryland Center for Environmental Science.